In a spiral antenna that radiates electromagnetic waves in the forward direction, a wideband characteristic and low profile are achieved by inserting a radio wave absorptive material into a space between the antenna and a cavity (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2000-252738).
If the lower limit of the operating frequency is lowered, however, the space between the cavity and spiral antenna physically widens, so the amount of radio wave absorptive material to be used increases. This makes it difficult to decrease the weight of the antenna.